U.S. Pat. No. 3,460,116 of A. H. Bobeck, U. F. Gianola, R. C. Sherwood, and W. Shockley; issued Aug. 5, 1969, discloses a magnetic bubble memory known as a "current-access" bubble memory. In a current access memory, bubbles are moved along paths defined by current carrying electrical conductors which produce a pattern of magnetic field gradients for effecting bubble movement in response to a succession of current pulses.
U.S. Pat. No. 3,618,051 of P. I. Bonyhard, U. F. Gianola, and A. J. Perneski; issued Nov. 2, 1971 describes a "field access" mode of operating bubble memories. A memory operating in the field-access mode utilizes a uniform magnetic field which rotates in the plane of bubble movement. The memory includes a periodic pattern of magnetically soft elements which align with the rotating field at varying orientations. Due to both the shapes of the elements and the succession of orientations, a succession of field gradients again are generated along a path and bubble movement occurs.
As is well known in the art, a field-access mode bubble memory employs permalloy to define a typical pattern of elements. However, alternatives are known. Such alternatives include grooves, mesas, and ion-implanted regions. The use of ion-implanted regions to produce bubble movement is disclosed in U.S. Pat. No. 3,792,452 of M. Dixon, R. A. Moline, J. C. North, L. J. Varnerin Jr., and R. Wolfe, issued Feb. 12, 1974.
As is further known in the art, a uniform ion implanted layer may be employed, along with permalloy propagation elements in a field-access mode magnetic bubble memory to suppress the formation of "hard bubbles." However, in a field-access magnetic bubble memory using a pattern of ion-implanted regions to produce bubble movement, no uniform ion-implanted layer is employed because bubbles in such circuits remain under the ion-implanted regions of the patterned layer. Hard bubble suppression by additional implantation in such circuits is superfluous.
It has been found difficult to make magnetic bubble generators with commercially acceptable characteristics combined with propagation elements formed by ion implantation. That is to say, magnetic bubble generators which utilized ion-implanted regions rather than permalloy typically generate bubbles in response to very low generator currents. In fact, generation occurs at such low currents that spurious bubbles appear in unwanted positions leading to low operating margins for current-controlled functions other than generation. Further, the propagation path may nucleate by itself leading to a loss of propagation margins.